1. Field of the Invention
The invention relates to improving the physical characteristics, such as the strength, the hardness, and the rain erosion durability, of chemically vapor-deposited zinc sulfide windows and hot isostatic pressed, chemically vapor-deposited zinc sulfide windows without compromising the optical transmission characteristics of such windows.
2. Brief Description of the Related Art
A chemically vapor-deposited zinc sulfide (hereafter "CVD ZnS") structure may be made by a variety of methods known in the art. One such method, for example, includes reacting gaseous zinc vapor (from molten zinc) and a gaseous hydrogen sulfide within a closed, growth chamber for a period sufficient to form a film of zinc sulfide on a mold within the chamber. The reaction period may be adjusted to achieve a zinc sulfide film of desired thickness. By cooling the film, a CVD ZnS structure is formed, the structure having a shape defined by the mold. The CVD ZnS structure is then removed from the chamber, machined to a desired size, ground, and polished to form a CVD ZnS window. See generally, J. A. Savage, Infrared Optical Materials and Their Antireflection Coatings, Adam Hilger LTD., Bristol and Boston (1985), pgs. 100-103. Optionally, the CVD ZnS window may be hot isostatic pressed (HIP) to improve the optical transmission characteristics of the window. Generally, CVD ZnS materials modified by a post-deposition hot isostatic press are commercially available, for example, under the tradename CLEARTRAN.RTM. from Morton International, Chicago, Ill.
Hot isostatic pressed CVD ZnS windows such as those made as CLEARTRAN.RTM. material are known for exhibiting excellent broadband transmission characteristics and have been used as window materials for optical and infrared applications. Hot isostatic pressed CVD ZnS windows, however, are soft and readily sustain damage causing loss in optical transmission when exposed to rain at high velocity. Such poor rain erosion durability limits the use of CLEARTRAN.RTM. material as a window in high velocity applications, such as in military aircraft.
Strength-enhancing coatings have been applied to increase the strength, the hardness, and the rain erosion durability of the formed hot isostatic pressed CVD ZnS windows. Unfortunately, however, application of such coatings to these windows has a detrimental effect on the optical transmission characteristics of the windows. For example, use of boron phosphide (BP), gallium phosphide (GaP), or gallium aluminum phosphide (GaAIP) coatings, while improving the strength, hardness, and rain erosion durability of hot isostatic pressed CVD ZnS windows, causes a degradation of transmission in the visible spectrum of light (e.g., about 380 nanometers to about 700 nanometers). Due to the detrimental effect such conventional, post-fabrication coatings have on hot isostatic pressed CVD ZnS windows, use of hot isostatic pressed CVD ZnS windows treated with such coatings may be limited to black and white video applications.
Another type of zinc sulfide window is made by powder processing techniques, as opposed to chemical vapor deposition techniques. Methods of increasing the strength of powder-processed zinc sulfide windows have included the preparation of a ZnS--Ga.sub.2 S.sub.3 solid solution by the in situ addition of gallium sulfide in a bulk zinc sulfide material. J. Zhang et al., Solid-State Phase Equilibria in the ZnS--Ga.sub.2 S.sub.3 System, J.AM. CERAMIC SOC'Y. Vol 73, No. [6], pgs. 1544-47 (1990). It has been shown that at certain temperatures, the gallium precipitates out of the ZnS--Ga.sub.2 S.sub.3 solution as a zinc thiogallate (ZnGa.sub.2 S.sub.4). W. W. Chen et al., Experimental and Theoretical Studies of Second-Phase Scattering in IR Transmitting ZnS-Based Windows, Proceedings of SPIE, San Diego (1991).
Another method of improving the strength of powder processed zinc sulfide windows includes a powder densification method disclosed in Harris et al., U.S. Pat. No. 5,575,959 and in Harris et al. U.S. Pat. No. 5,643,505. These patents teach the addition of gallium for hardening zinc sulfide by co-precipitation of the gallium as an integral part of the zinc sulfide crystal lattice which forms an intimate mixture of zinc sulfide and gallium sulfide (ZnS--Ga.sub.2 S.sub.3). The ZnS--Ga.sub.2 S.sub.3 solid solution can then be mixed with pure zinc sulfide as a bulk component, a surface-enrichment component, or as a gradient of concentrations. The resulting zinc thiogallate (ZnGa.sub.2 S.sub.4) precipitate has the effect of hardening and strengthening the zinc sulfide window. A subsequent processing step includes hot isostatic pressing.
In an alternative method disclosed in the aforementioned patents, gallium metal is evaporated onto the surface of a powder processed zinc sulfide body, which is formed by co-precipitation of the sulfide and zinc salt, and subsequent hot pressing and hot isostatic pressing of the co-precipitate. Once the gallium metal has been evaporated onto the surface of the zinc sulfide body, the body is annealed to diffuse the gallium metal into the zinc sulfide body and, following subsequent suitable annealing, a surface zinc thiogallate phase (ZnGa.sub.2 S.sub.4) is formed.
To the extent that the methods disclosed in the prior art (e.g., the above-identified Harris et al. patents) have resulted in measurable improvements in infrared transmission and hardness, such methods are limited to powder processing and do not teach how to harden CVD ZnS materials or hot isostatic pressed CVD ZnS materials. For example, attempts to evaporate gallium metal into the surface of CVD ZnS and the subsequent annealing thereof to effect diffusion of the gallium metal have proven difficult, and often inadequate. Furthermore, the prior art is silent as to how to improve the strength, the hardness, and the rain erosion durability of commercially-available CVD ZnS windows or hot isostatic pressed CVD ZnS windows without compromising the desirable optical transmission characteristics of the windows. Accordingly, it would be desirable to provide a method of improving the rain erosion durability of CVD ZnS windows and hot isostatic pressed CVD ZnS windows, without detrimentally influencing the optical transmission characteristics of the windows.